In recent years, due to the constant increase of energy prices dictated, in particular, by the constant increase in the price of fossil fuels, more and more interest has been shown toward renewable energy sources. One particularly appreciated form of renewable energy consists of photovoltaic energy, which can be installed virtually anywhere in sizes ranging from a few square centimeters to photovoltaic parts covering several square kilometers.
One particularly advantageous form of photovoltaic generators consists of III-V concentrator generators. Even more specifically, multi-junction III-V concentrator photovoltaic cells are preferred since they can achieve much higher efficiencies compared to the standard silicon technology. In the concentrator photovoltaic module, the size of the photovoltaic cells is reduced thanks to the use of a lens focusing light from a larger area to a smaller area, corresponding to the smaller size of the photovoltaic cells. This implies that the photovoltaic cells do not cover the entire area of the module, but are rather spaced from each other. Depending on the concentration of the light, the use of a heat sink might be necessary in order to avoid efficiency losses due to increased temperature of the photovoltaic cells. Therefore, when realizing the module, it is a standard approach to pick and place each of the cells, at a certain distance from each other, for instance, on top of the heat sink as part of the concentrator photovoltaic module.
Such a standard approach is illustrated in FIG. 6. In particular, FIG. 6 illustrates a plurality of cut views of the product during different steps of the manufacturing process. A substrate 8130 is provided on which, during a step S81, a photovoltaic layer 8120 is provided. For instance, the photovoltaic layer 8120 could be grown by epitaxial growth of III-V semiconducting materials like indium gallium phosphide and gallium arsenide on, for example, a germanium wafer, leading to a common multi junction solar cell of type InGaP/GaAs/Ge. The germanium wafer could typically have a diameter of 4, 6, or 8 inches. The photovoltaic layer 8120 could further include an anti-reflective coating and metal contacts on the front and/or rear side. During a step S82, the photovoltaic layer 8120 is cut so as to realize a plurality of photovoltaic cells 8121-8124 separated from each other. The substrate 8130 has to serve as a stabilizing support for the photovoltaic layer 8120. In the case of the above-mentioned InGaP/GaAs/Ge multi junction layer, the photovoltaic layer 8120 comprising the germanium wafer and the epitaxially grown semiconducting layers are provided on an adhesive foil, in particular, a typically blue foil with adhesive, which reduces adhesion after UV exposure, acting as substrate 8130 in order to maintain the positions of the photovoltaic cells 8121-8124 after cutting/separation. The germanium wafer has to have a minimum thickness in order to allow a subsequent pick and place of the cells. Part of that germanium wafer forms the bottom junction of the InGaP/GaAs/Ge multi-junction, whereas the rest of the Germanium substrate acts as stiffener for the separated photovoltaic cells. Such a separation or cut could be achieved by etching, diamond cut, sawing, laser separation, or any other technique used in the field of photovoltaic manufacturing and, more generally, semiconductor technology. During a step S83, the photovoltaic cells 8121-8124 are picked and placed, one by one, from substrate 8130 onto a heat sink 8140, thereby realizing structure 8101. Thereby the application of conductive glue or solder 8131-8134 is necessary in order to secure each individual photovoltaic cell on the heat sink 8140. One could also imagine several individual heat sinks 8140 for each individual photovoltaic cell 8121-8124. Heat sinks could be made of copper, aluminum or other metals and contain further elements like bypass diodes or contact pads. Structure 8101 can thereafter be placed into a module, with the addition of a lens layer (not illustrated), above the cells 8121-8124.
Such an approach is, however, slow and expensive, mainly due to the fact that the photovoltaic cells 8121-8124 have to be singularly, one by one, picked and placed from substrate 8130 to heat sink 8140. The pick-and-place process can be manual, or automated, but it remains a serial process, which is complicated and slow. The process does not allow all cells on one wafer to be processed simultaneously. Further, a sort of pick-and-place process has been developed by Semprius, as disclosed in WO2011/123285A1, and can be used to transfer a multiple number of cells by selectively bonding widely spaced cells from one wafer and releasing them to another structure at their initial separation distance. In this process, not all the cells from the starting wafer are processed simultaneously, neither a change of their distance separating them from each other is disclosed.
Additionally, the pick and pick-and-place approach is not satisfactory since the cells have to be manufactured with a certain thickness to provide sufficient stiffness in order to be manipulated. Further, in case of the common InGaP/GaAs/Ge cells, the initial thick Germanium substrate is rather expensive. Further, thick layers of the photovoltaic layer 8120 can cause losses due to high resistance for current and/or heat conduction. Thin layers with sufficient conductivity are preferred to minimize such losses. A thick layer of solder or conductive glue 8131-8134 is used to connect the semiconductor layers 8121-8124 to the heat sink 8140. Also, this layer causes additional resistance losses and leads to concerns of reliability due to different thermal expansion and corrosion, which can damage the interconnect over time.